Sensing of the signals resulting from atrial cardiac events is well known. Any of a number of detection systems that include the conventional discrete circuits for signal acquisition, conditioning and typically conversion to digital values followed by digital signal processing for detecting electrical activity in the heart, including the atrium, can be used. These detection systems are also typically referred to as detection circuits. In particular, the sensitivity of a detection circuit for detecting the signals resulting from the atrial cavity is programmed to a sensitivity value that is considered high, i.e., a threshold for the detection of atrial events that is relatively low (typically about 0.4 mV). This high sensitivity (low detection threshold) is required to be able to sense and to interpret low amplitude signals in the atrium in the case of disorders of the atrial rate. The term “disorders of the atrial rate” is a generic term that covers a variety of atrial arrhythmias such as atrial tachycardia, atrial fibrillation, atrial flutter, etc.; disturbances that are characterized, during the detection, by an abnormal and fast atrial rate. In conventional cardiac detection circuits, the atrial sensitivity is a programmable parameter, but is generally fixed at a constant value, at least during the same cardiac cycle.
The high sensitivity necessarily results in false detections (known as false positives), i.e., a detection in the atrial cavity of signals not corresponding to a spontaneous atrial event (that is, a spontaneous depolarization of the myocardium in the atrium, also called a “detected atrial event”) or a stimulated atrial event (that is, a depolarization caused by an application of a stimulation pulse on the atrium, also called a “paced atrial event”). Typically, these false positive detections (also known as parasitic detections) result from a phenomenon known by the name of “far-field” or “far-field sensing” (“FFS”), that occurs when the pacemaker detects in the atrial cavity a signal resulting from a prior ventricular depolarization. In other words, the depolarization of the ventricle is propagated to the atrium and, although that signal is attenuated, its amplitude remains greater than the detection threshold and thus, comes to delude the atrial detection circuit.
A disadvantage of such an FFS detection is that it can be interpreted by the device as an atrial extrasystole. If this phenomenon is repetitive, the apparatus may interpret it as an atrial tachycardia and consequently improperly start algorithms to prevent or treat the supposed (but non-existent) atrial tachycardia. In other words, the false positive detection(s) can lead to a false diagnosis of “association” when the device detects—wrongly—atrial events (as a consequence of the far-field) as often as ventricular events (at the origin of the far-field). This phenomenon strongly penalizes the detection of a “dissociation” of the cardiac rate (i.e., when the atrial events are desynchronized and fewer in number than the ventricular events) and, in a general way, the detection of the arrhythmias of all kinds.
Devices able to provide such a modification of the sensitivity of the atrial detection circuit after an atrial event are known, such as the DEFENDERr™ or ALTO™ implantable devices marketed by Ela Médical, Montrouge France or as described in the publication WO-A-00/47277 and its corresponding U.S. Pat. 6,249,701. However, these devices operate in an undifferentiated manner that does not make it possible to eliminate the phenomenon of far-field detection nor to take into account the consequences that a ventricular extrasystole can have on the operation of these devices.